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  1. 1. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704 Power Upgrading of Transmission Line by Combining AC–DC Transmission P. Vijay Kumar Babu*, P. Bhaskara Prasad ** , M.Padma Lalitha*** *(P.G Student, Department of Electrical Engg, AITS College, Rajampeta,) ** (Assistant professor, Department of Electrical Engg, AITS college, Rajampeta,) *** (Professor, Department of Electrical Engg, AITS college, Rajampeta,)ABSTRACT Long extra high voltage (EHV) ac lines full utilization of existing transmission facilitiescannot be loaded to their thermal limits in order without decreasing system availability and security.to keep sufficient margin against transient The flexible ac transmission system (FACTS)instability. With the scheme proposed in this concepts, based on applying state-of-the-art powerproject, it is possible to load these lines very close electronic technology to existing ac transmissionto their thermal limits. The conductors are system, improve stability to achieve powerallowed to carry usual ac along with dc transmission close to its thermal limit. Another waysuperimposed on it. The added dc power flow to achieve the same goal is simultaneous ac–dcdoes not cause any transient instability. This power transmission in which the conductors areproject gives the feasibility of converting a allowed to carry superimposed dc current along withdouble circuit ac line into composite ac–dc power ac current. Ac and dc power flow independently,transmission line to get the advantages of parallel and the added dc power flow does not cause anyac–dc transmission to improve stability and transient instability.damping out oscillations. Simulation and Simultaneous ac–dc power transmissionexperimental studies are carried out for the was first proposed through a single circuit accoordinated control as well as independent transmission line. In these proposals Mono-polar dccontrol of ac and dc power transmissions. No transmission with ground as return path was used.alterations of conductors, insulator strings, and There were certain limitations due to use of groundtowers of the original line are needed. Substantial as return path. Moreover, the instantaneous value ofgain in the load ability of the line is obtained. each conductor voltage with respect to groundMaster current controller senses ac current and becomes higher by the amount of the dc voltage,regulates the dc current orders for converters and more discs are to be added in each insulatoronline such that conductor current never exceeds string to withstand this increased voltage. However,its thermal limit. there was no change in the conductor separation distance, as the line-to-line voltage remainsIndex Terms— Extra high voltage (EHV) unchanged. The feasibility study of conversion of atransmission, flexible ac transmission system double circuit ac line to composite ac–dc line(FACTS), power system computer-aided design without altering the original line conductors, tower(PSCAD) simulation, simultaneous ac–dc power structures, and insulator strings has been presented.transmission. In this scheme, the dc power flow is point-to- point bipolar transmission system.I. INTRODUCTION In Recent years, environmental, right-of- II.SIMULTANEOUS AC–DC POWERway, and cost concerns have delayed the TRANSMISSIONconstruction of a new transmission line, while A. Basic scheme for composite ac–dcdemand of electric power has shown steady but transmission.geographically uneven growth. The power is often The dc power is obtained through lineavailable at locations not close to the growing load commutated 12-pulse rectifier bridge used incenters but at remote locations. These locations are conventional HVDC and injected to the neutrallargely determined by regulatory policies, point of the zigzag connected secondary of sendingenvironmental acceptability, and the cost of end transformer and is reconverted to ac again byavailable energy. The wheeling of this available the conventional line commutated 12-pulse bridgeenergy through existing long ac lines to load centers inverter at the receiving end. The inverter bridge ishas a certain upper limit due to stability again connected to the neutral of zig-zag connectedconsiderations. Thus, these lines are not loaded to winding of the receiving end transformer.their thermal limit to keep sufficient margin againsttransient instability. The present situation demandsthe review of traditional power transmission theoryand practice, on the basis of new concepts that allow 1|Page
  2. 2. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704 for active and reactive powers in terms of A, B, C, and D parameters of each line may be written as ES=AER+BIR (1) (2) IS=CER+DIR * * 2 * (3) PS+JQS= - ES*ER/B +D ES /B PR+jQR= ES*ER/B*-A* ER2/B*. (4) Fig. 1 depicts the basic scheme for simultaneousac–dc power flow through a double circuit actransmission line. The double circuit ac transmission linecarriers both three-phase ac and dc power. Eachconductor of each line carries one third of the totaldc current along with ac current. Resistance beingequal in all the three phases of secondary winding ofzig-zag transformer as well as the three conductorsof the line, the dc current is equally divided amongall the three phases. The three conductors of thesecond line provide return path for the dc current. III. DESCRIPTION OF THE SYSTEMZig-zag connected winding is used at both ends to MODELavoid saturation of transformer due to dc current. A. Description of the system model:Two fluxes produced by the dc current Id/3 flowing A synchronous machine is feeding powerthrough each of a winding in each limb of the core to infinite bus via a double circuit,three-of a zig-zag transformer are equal in magnitude and phase,400KV,50Hz, 450Km ac transmission line.opposite in direction. So the net dc flux at any The 2750MVA,24KV synchronous machine isinstant of time becomes zero in each limb of the dynamically modeled, a field coil on d-axis and acore. Thus, the dc saturation of the core is avoided. damper coil on q-axis, by Park’s equations with theA high value of reactor Xd is used to reduce frame of reference based in rotor. It is equipped withharmonics in dc current. In the absence of zero an IEEE type AC4A excitation system of whichsequence and third harmonics or its multiple block diagram is shown in Fig. 3. Transmissionharmonic voltages, under normal operating lines are represented as the Bergeron model. It isconditions, the ac current flow through each based on a distributed LC parameter traveling wavetransmission line will be restricted between the line model, with lumped resistance. It represents thezigzag connected windings and the three conductors L and C elements of a PI section in a distributedof the transmission line. Even the presence of these manner (i.e., it does not use lumped parameters).components of voltages may only be able to produce It is roughly equivalent to using an infinitenegligible current through the ground due to high number of PI sections, except that the resistance isvalue of Xd. lumped (1/2 in the middle of the line, 1/4 at each Assuming the usual constant current end). Like PI sections, the Bergeron modelcontrol of rectifier and constant extinction angle accurately represents the fundamental frequencycontrol of inverter, the equivalent circuit of the only. It also represents impedances at otherscheme under normal steady-state operating frequencies, except that the losses do not change.condition is given in Fig. 2. The dotted lines in the This model is suitable for studies where thefigure show the path of ac return current only. The fundamental frequency load flow is most important.second transmission line carries the return dc The converters on each end of dc link are modeledcurrent, and each conductor of the line carries Id/3 as line commutated two six- pulse bridge (12-pulse),along with the ac current per phase. And are the Their control system consist of constant currentmaximum values of rectifier and inverter side dc (CC) and constant extinction angle (CEA) andvoltages and are parameters per phase of each line. voltage dependent current order limiters (VDCOL)Rcr, Rci are commutating resistances, and, α, γ are control. The converters are connected to ac busesfiring and extinction angles of rectifier and inverter, via Y-Y and Y- converter transformers. Each bridgerespectively. Neglecting the resistive drops in the is a compact power system computer-aided designline conductors and transformer windings due to dc (SIMULINK) representation of a dc converter,current, expressions for ac voltage and current, and which includes a built in six-pulse Graetz converter bridge (can be inverter or rectifier), an internal 1700 | P a g e
  3. 3. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704phase locked oscillator (PLO), firing and valve A. Technology involved in an HVDC system:blocking controls, and firing angle /extinction α There are three ways of achieving conversionangle γ measurements. It also includes built in RC 1. Natural commutated converterssnubber circuits for each thyristor. The controls used 2. Capacitor Commutated Convertersin dc system are those of CIGRE Benchmark, 3. Forced Commutated Convertersmodified to suit at desired dc voltage. Ac filters ateach end on ac sides of converter transformers areconnected to filter out 11th and 13th harmonics. 1. Natural commutated converter: (NCC):These filters and shunt capacitor supply reactive NCC are most used in the HVDC systemspower requirements of converters. as of today. The component that enables this A master current controller (MCC), shown conversion process is the thyristor, which is ain Fig. 4, is used to control the current order for controllable semiconductor that can carry very highconverters. It measures the conductor ac current, currents (4000 A) and is able to block very highcomputes the permissible dc current, and produces voltages (up to 10 kV). By means of connecting thedc current order for inverters and rectifiers. thyristors in series it is possible to build up a thyristor valve, which is able to operate at very high voltages (several hundred of kV).The thyristor valve is operated at net frequency (50 Hz or 60 Hz) and by means of a control angle it is possible to change the DC voltage level of the bridge. 2. Capacitor Commutated Converters (CCC): An improvement in the thyristor-based Commutation, the CCC concept is characterized by the use of commutation capacitors inserted in series between the converter transformers and the thyristor valves. The commutation capacitors improve the commutation failure performance of the convertersIV. HIGH VOLTAGE DC when connected to weak networks.TRANSMISSION This chapter describes an overview of the 3. Forced Commutated Converters:importance of the HVDC systems in the present This type of converters introduces aworld. Then it discusses about the various spectrum of advantages, e.g. feed of passiveparameters dealing with the HVDC systems. Over networks (without generation), independent controllong distances bulk power transfer can be carried of active and reactive power, power quality. Theout by a high voltage direct current (HVDC) valves of these converters are built up itconnection cheaper than by a long distance AC semiconductors with the ability not only to turn-ontransmission line. HVDC transmission can also be but also to turn-off. They are known as Voltageused where an AC transmission scheme could not Source Converters (VSC). Two types of(e.g. through very long cables or across borders semiconductors are normally used in voltage sourcewhere the two AC systems are not synchronized or converters: the GTO (Gate Turn-Off Thyristor) oroperating at the same frequency). However, in order the IGBT (Insulated Gate Bipolar Transistor). Bothto achieve these long distance transmission links, of them have been in frequent use in industrialpower converter equipment is required, which is a application, since the early eighties. The VSCpossible point of failure and any interruption in commutates with high frequency (not with the netdelivered power can be costly. It is therefore of frequency). The operation of the converter iscritical importance to design a HVDC scheme for a achieved by Pulse Width Modulation (PWM).Withgiven availability. The HVDC technology is a high PWM it is possible to create any phase angle orpower electronics technology used in electric power amplitude (up to certain limit) by changing thesystems. It is an efficient and flexible method to PWM pattern, which can be done almosttransmit large amounts of electric power over long instantaneously. Thus, PWM offers the possibility todistances by overhead transmission lines or control both active and reactive powerunderground/submarine cables. It can also be used independently. This makes the PWM Voltageto interconnect asynchronous power systems. The Source Converter a close to ideal component in thefundamental process that occurs in an HVDC transmission network. From a transmission networksystem is the conversion of electrical current from viewpoint, it acts as a motor or generator withoutAC to DC (rectifier) at the transmitting end and mass that can control active and reactive powerfrom DC to AC (inverter) at the receiving end. almost instantaneously. 1701 | P a g e
  4. 4. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704B. Configurations of HVDC: 4. Multi-terminal HVDC system:There are different types of HVDC systems whichare1. Mono-polar HVDC system: In the mono-polar configuration, twoconverters are connected by a single pole line and apositive or a negative DC voltage is used. In Fig.There is only one Insulated transmission conductorinstalled and the ground or sea provides the path forthe return current. In the multi terminal configuration, three or more HVDC converter stations are geographically2. Bipolar HVDC system: separated and interconnected through transmission This is the most commonly used lines or cables. The System can be either parallel,configuration of HVDC transmission systems. The where all converter stations are connected to thebipolar configuration, shown in Fig. Uses two same voltage as shown in Fig(b). or seriesinsulated conductors as Positive and negative poles. multiterminal system, where one or more converterThe two poles can be operated independently if both stations are connected in series in one or both polesNeutrals are grounded. The bipolar configuration as shown in Fig. (c). A hybrid multiterminal systemincreases the power transfer capacity. Under normal contains a combination of parallel and seriesoperation, the currents flowing in both poles are connections of converter stations.identical and there is no ground current. In case offailure of one pole power transmission can continue C. DC transmission control:in the other pole which increases the reliability. The current flowing in the DC transmissionMost overhead line HVDC transmission systems use line shown in Figure below is determined by the DCthe bipolar configuration. voltage difference between station A and station B. Using the notation shown in the figure, where rd represents the total resistance of the line, we get for the DC current And the power transmitted into station B is3. Homo-polar HVDC system: In the homo polar configuration, shown inFig. Two or more conductors have the negativepolarity and can be operated with ground or a In rectifier operation the firing angle αmetallic return. With two Poles operated in parallel, should not be decreased below a certain minimumthe homo polar configuration reduces the insulation value αmin normally 3°-5° in order to make sure thatcosts. However, the large earth return current is the there really is a positive voltage across the valve atmajor disadvantage. the firing instant. In inverter operation the extinction angle should never decrease below a certain minimum value γmin, normally 17°-19° otherwise the risk of commutation failures becomes too high. On the other hand, both α and γ should be as low as possible to keep the necessary nominal rating of the equipment to a minimum. Low values of α and γ also decrease the consumption of reactive power and the harmonic distortion in the AC networks. To achieve this, most HVDC systems are controlled to maintain γ= γmin, in normal operation. The DC 1702 | P a g e
  5. 5. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704voltage level is controlled by the transformer tap these signals will influence the dynamics of thechanger in inverter station B. The DC current is total control system.controlled by varying the DC voltage in rectifierstation A, and thereby the voltage difference V. SIMULATION RESULTSbetween A and B. Due to the small DC resistances The performance of the proposed controlin such a system, only a small voltage difference is system was evaluated with a detailed simulationrequired, and small variations in rectifier voltage model using the MATLAB/Simulink,gives large variations in current and transmitted SimPowerSystems to represent the master control,power. The DC current through a converter cannot transformers, sources and transmission lines, andchange the direction of flow. So the only way to Simulink blocks to simulate the control system.change the direction of power flow through a DCtransmission line is to reverse the voltage of the line. Combined ac dc current and ac current in zigzagBut the sign of the voltage difference has to be kept transformer:-constantly positive to keep the current flowing. Tokeep the firing angle α as low as possible, thetransformer tap changer in rectifier station A isoperated to keep α on an operating value whichgives only the necessary margin to αmin to be ableto control the current.D. Master control system: The master control, however, is usuallysystem specific and individually designed.Depending on the requirements of the transmission,the control can be designed for constant current orconstant power transmitted, or it can be designed tohelp stabilizing the frequency in one of the ACnetworks by varying the amount of active power Rectifier ac voltage and current:-transmitted. The control systems are normallyidentical in both converter systems in atransmission, but the master control is only active inthe station selected to act as the master station,which controls the current command. The calculatedcurrent command is transmitted by a communicationsystem to the slave converter station, where the pre-designed current margin is added if the slave is toact as rectifier, subtracted if it is to act as inverter. Inorder to synchronize the two converters and assurethat they operate with same current command, atele-communications channel is required. Sending end voltage:- Should the telecommunications system fail for any reason, the current commands to both converters are frozen, thus allowing the transmission to stay in operation. The requirements for the telecommunications system are especially high if the transmission is required to have a fast control of the transmitted power, Receiving end voltage:- and the time delay in processing and transmitting 1703 | P a g e
  6. 6. P. Vijay Kumar Babu, P. Bhaskara Prasad, M.Padma Lalitha / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 6, November- December 2012, pp.1699-1704 [8] Padiyar, HVDC Power Transmission System. New Delhi, India: Wiley Eastern, 1993. [9] E. W. Kimbark, Direct Current Transmission. New York: Wiley, 1971, vol. I. [10] J. Arillaga and N. R.Watson, Computer Modelling of Electrical Power Systems. Chichester, U.K.: Wiley, 2003.VI. CONCLUSION The feasibility to convert actransmission line to a composite ac–dc line has beendemonstrated. For the particular system studied,there is substantial increase (about 83.45%) in theload ability of the line. The line is loaded to itsthermal limit with the superimposed dc current. Thedc power flow does not impose any stabilityproblem. The advantage of parallel ac–dctransmission is obtained. Dc current regulator maymodulate ac power flow. There is no need for anymodification in the size of conductors, insulatorstrings, and towers structure of the original line. Theoptimum values of ac and dc voltage components ofthe converted composite line are 1/2 and 1/√2 timesthe ac voltage before conversion, respectively.REFERENCES [1] L. K. Gyugyi, ―Unified power flow concept for flexible A.C. transmission system,‖ Proc. Inst. Elect. Eng., p. 323, Jul. 1992. [2] L. K. Gyugyi et al., ―The unified power flow controller; a new approach to power transmission control,‖ IEEE Trans. Power Del., vol. 10, no. 2, pp. 1085–1097, Apr. 1995. [3] N. G. Hingorani, ―FACTS—flexible A.C. transmission system,‖ in Proc. Inst. Elect. Eng. 5th. Int. Conf. A.C. D.C. Power Transmission, London, U.K., 1991. [4] P. S. Kundur, Power System Stability and Control. New York: Mc- Graw-Hill, 1994. [5] K. P. Basu and B. H. Khan, ―Simultaneous ac-dc power transmission,‖ Inst. Eng. (India) J.-EL, vol. 82, pp. 32–35, Jun. 2001. [6] H. Rahman and B. H. Khan, ―Enhanced power transfer by Simultaneous transmission of AC-DC: a new FACTS concept,‖ in Proc. Inst. Elect. Eng. Conf. Power Electronics, Machines, Drives, Edinburgh, U.K., Mar. 31– Apr. 2 2004, vol. 1, pp. 186–191. [7] A. Clerici, L. Paris, and P. Danfors, ―HVDC conversion of HVAC line to provide substantial power upgrading,‖ IEEE Trans. Power Del., vol. 6, no. 1, pp. 324–333, Jan. 1991. 1704 | P a g e